Inter-area modes of oscillation are typically characterised by a group of machines in one geographical area of a power transmission system swinging against a group of machines in another geographical area of the system. Inter-area modes of oscillation is for instance described in CN 101202451, U.S. Pat. No. 6,252,753 and EP 1852952.
These oscillations are initiated by e.g. normal changes in the system load or switching events in the system possibly following faults. These oscillations may typically have a frequency of less than a few Hz, for instance in the range of 0.1-0.8 Hz, and are often considered acceptable as long as they decay fast enough. Insufficiently damped oscillations may occur when the operating point of the power system is changed, for example, due to a new distribution of power flows following a connection or disconnection of generators, loads and/or transmission lines. In these cases, an increase in the transmitted power of a few MW may make the difference between stable oscillations and unstable oscillations which have the potential to cause a system collapse or result in loss of synchronism, loss of interconnections and ultimately the inability to supply electric power to customers. Appropriate monitoring and control of the power transmission system can help a network operator to accurately assess power transmission system states and avoid a total blackout by taking appropriate actions such as the connection of specially designed oscillation damping equipment.
The conventional way to perform Power Oscillation Damping (POD) is by adding a modulation signal to the control signal of an actuator which counteracts the power oscillation. Typical actuators which could be controlled to perform such damping include synchronous generators, HVDC and FACTS installations.
There are different ways in which such oscillations can be dampened. One way is through using a POD device that employs lead-lag compensation.
Another way in which power oscillation damping can be performed is through the use of phasor based damping in a so-called phasor POD. A phasor POD is described in U.S. Pat. No. 6,559,561. In a phasor POD an auxiliary signal is provided to a power flow controller or voltage controller for actuators in the power transmission system in order to damp such oscillation. The phasor POD uses a scheme which expresses the active power, voltage or current oscillation in a rotating coordinate system and control action is synthesized in another phasor form to counteract the root oscillation. In this process, the knowledge of the system oscillation frequency, optimal phase difference of control signal with respect to measured signal and appropriate gain is needed a-priori. Thus with this technique an appropriate compensation angle needs to be known a-priori for each operating condition.
However there is a problem associated with using a fixed phase compensation angle in the above-described way. The configuration of a power transmission system may change, for instance because of a line outage following a severe fault. This means that different phase angles may be needed for different conditions. In order to provide efficient damping after a fault it may then be necessary to determine the operating condition after the fault. There is today no existing technique for determining such a post-disturbance operating condition.
There is therefore a need for improvement in this field of technology.